1. Field of the Invention
The present invention relates to a hybrid lens and a projection optical system using the same, and more particularly, to a hybrid lens capable of realizing a high resolution and a projection optical system using the same.
2. Description of the Related Art
Focusing performance is very important in a projection TV optical system, which projects an original image formed on a fluorescent face of a Braun tube (CRT; Cathode Ray Tube) onto a screen. For focusing, a projection optical system uses an aspherical plastic lens capable of compensating for aberration. However, the refractive index, shape, and focal length of the aspherical plastic lens vary with changes in temperature, which degrades the focusing performance. In order to solve these problems, in the prior art, including WO 98-34134, U.S. Pat. Nos. 5,272,540, and 4,924,244, a projection optical system using a technique for combining a single lens, a double glass lens, and an aspherical lens is proposed.
FIG. 1 is a cross-sectional view of a projection optical system disclosed in U.S. Pat. No. 5,272,540. Here, reference numerals 11, 12, 13, 14, and 15 denote first, second, third, fourth, and fifth group lenses, respectively. Here, the third group lens 13 is composed of a pair of glass lenses 13a and 13b. Reference numeral 16 denotes a cooling liquid, reference numeral 17 denotes a protective lens that covers a fluorescent surface P1 of a CRT, and reference numeral 19 denotes a screen. Referring to FIG. 1, RAY1 denotes an upper limit and RAY2 denotes a lower limit of light rays emitted from a center point A of the fluorescent surface P1 of the CRT. RAY3 denotes an upper limit and RAY4 denotes a lower limit of light rays emitted from an object point B in the periphery of an image plane.
The first and second group lenses 11 and 12 are aspherical plastic lenses positioned on an optical axis to provide weak, positive refractive index. In order to reduce the dependence of the focus drift on temperature, the first group lens 11 is made concave and disposed adjacent to the screen 19, the second group lens 12 is made convex and disposed behind the first group lens 11, and the refractive indexes of the first and second group lenses 11 and 12 are substantially the same. The third group lens 13 is formed by bonding a pair of glass lenses and is disposed immediately behind the second group lens 12.
However, it is costly to manufacture groups of lenses constituting the projection optical system disclosed in the prior art, including U.S. Pat. No. 5,272,540, and image quality is degraded.
FIG. 2 is a graph of a modulation transfer function (MTF) versus a spatial frequency in cycles per millimeter when a general aspherical lens is used. Here, the MTF is defined by Equation 1 in terms of the maximum intensity Max and the minimum intensity Min of light. When the MTF is 1, resolution is optimal. Resolution decreases with a reduction in the MTF.                               M          ⁢                                          ⁢          T          ⁢                                          ⁢          F                =                              Max            -            Min                                Max            +            Min                                              (        1        )            
Referring to FIG. 2, f0 represents a case when an image height h (which refers to a distance from a point “O” where the optical axis meets an image plane to an image) is zero, f1 represents a case when the image height h is 20 mm, f2 represents a case when the image height h is 40 mm, f2 represents a case when the image height h is 60 mm, and f4 represents a case the image height h is 63.50 mm. Regardless of the variation in the spatial frequency, the MTF is the highest when the image height h is zero (f0) and the lowest when the image height h is 40 mm (f2). However, as can be seen from f0, f1, f2, f3, and f4, the MTF sharply decreases with an increase in the spatial frequency. As the MTF decreases, the contrast of an image formed by the projection optical system is reduced.
FIGS. 3A through 3C respectively illustrate the emission spectrums of a CRT emitting green, blue, and red light. Referring to FIG. 3A, the emission intensity of green light has a primary peak at a wavelength of 550 nm, a secondary peak at a wavelength of 490 nm, and is weak at other wavelengths. Referring to FIG. 3B, the emission intensity of blue light peaks at a wavelength of 450 nm and falls off within a wavelength range of 400 nm–500 nm. Referring to FIG. 3C, the emission intensity of red light peaks at a wavelength of 620 nm. Since the emission spectra of blue and red light overlap around the central wavelength of green light, the wavelengths of blue and red light, except the central wavelength of green light, are emitted in different wavelength bands, which results in chromatic aberration. Thus, an optical system capable of reducing the chromatic aberration is required.